1. Technical Field
The present disclosure relates to a power transmission device and an electric power transfer system, and, in particular, to a power transmission device that contactlessly transmits electric power to a power receiving device, and an electric power transfer system including the power transmission device.
2. Description of Related Art
Electric power transfer systems each configured to contactlessly transfer electric power from a power transmission device to a power receiving device are known (see, for example, Japanese Patent Application Publications No. 2014-207795 (JP 2014-207795 A), No. 2013-154815 (JP 2013-154815 A), No. 2013-146154 (JP 2013-146154 A), No. 2013-146148 (JP 2013-146148 A), No. 2013-110822 (JP 2013-110822 A), and No. 2013-126327 (JP 2013-126327 A)). As one example of the electric power transfer systems, JP 2014-207795 A discloses a contactless power feed system that contactlessly feeds electric power from a power feed device (power transmission device) to a vehicle (power receiving device). In the contactless power feed system, the power feed device includes a power transmission coil, an inverter, and a controller. The power transmission coil contactlessly transmits electric power to a power receiving coil installed on the vehicle. The inverter produces alternate current according to the drive frequency, and delivers the current to the power transmission coil. The controller obtains a command of charging power to a battery and output power to the battery, from the vehicle side, and controls the drive frequency of the inverter in a feedback manner, so that the output power follows the charging power command (see JP 2014-207795 A).
As in the contactless power feed system described in JP 2014-207795 A, it is possible to control the magnitude of electric power to be transmitted (which will be called “transmission power”), by adjusting the frequency, and thus ensure the capability of following desired electric power (which will also be called “transmission power control”).
By controlling the frequency, it is also possible to control the phase of the output current of the inverter relative to that of the output voltage of the inverter. In the inverter of voltage type, if the phase of the current leads the phase of the voltage, recovery current flows through a flyback diode of the inverter, and the inverter may break down. Thus, when the phase of the current lead the phase of the voltage, the inverter can be protected by adjusting the frequency in such a direction as to delay the phase of the current. In this connection, the lead of the current phase relative to the voltage phase can be detected, depending on whether positive current (positive turn-on current) flows when the voltage rises. Accordingly, the frequency can be adjusted in the direction to delay the current phase, based on a detection value of the turn-on current. In the following description, the control based on the detection value of the turn-on current will also be called “turn-on current control”.
Further, it is possible to control current flowing through the power transmission coil under a condition that electric power is maintained, by adjusting the frequency. Thus, the frequency is adjusted so that the current flowing through the power transmission coil is minimized, whereby the efficiency of electric power transfer between the power transmission coil and the power receiving coil can be enhanced. In the following description, this control will also be called “power transmission coil current control”.
Each of the above-described controls is performed by manipulating or controlling the frequency. Therefore, when the above-described three controls are implemented at the same time, the output (frequency manipulation) of certain control may have an adverse influence on the purpose of another control. However, component protection (protection of the inverter) to be achieved by the turn-on current control, as one of the purposes of the above controls, should be prioritized over the electric power following by the transmission power control, and the improvement of the power transfer efficiency by the power transmission coil current control.